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The cost for each installation was based on the bid price of the contractor and the
amount of each type of control observed during the inventory. The bid prices are
preconstruction estimates and the actual cost could have been either higher or lower
depending on the actual quantity of material used in the specific job. The cost per area
drained was calculated for each control type to compare the cost effectiveness of the
different types of controls. For example, the cost per hectare drained for both silt fences
and erosion control blankets can be compared; however, a comparison of the meters of
silt fence to square meters of erosion control blankets is not relevant.
3.2 Inventory Results
The inventory of temporary runoff controls was conducted between October 1993
and January 1994. This short time span allowed only a glimpse of some of the controls
that might be used during the life of a highway construction project. The fact that there
were six active sites in the study area permitted some diversity in the phases of
construction observed.
The quantities, drainage areas, and costs of runoff controls at the six sites
inventoried are summarized in Table 3.2. Rock berms were used most commonly in the
study area, treating drainage from 53% of the area of the six sites. Silt fences and
sedimentation ponds were runoff controls used on 23% and 22% of the total area,
respectively. Erosion control blankets (ECB) were used over 6% of the study area. These
results are shown graphically in Figure 3.2.
Construction activities at sites on State Highway 45, Loop 1, Loop 360 and
Westbank Dr., and the hazardous material traps (HMT's) at Loop 1 and Gaines Creek
were near completion. The projects at the US 290 and Loop 360 interchange and on RM
967 were in earlier stages of construction. The data presented in Figure 3.3 show that the
sites with construction activities nearing completion relied on erosion control blankets
much more than sites in the earlier stages of construction, since erosion control blankets
are typically placed on surfaces after the final grading is complete. Sediment ponds
appeared to be used more on sites that are in the earlier stages of construction.
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